All ecosystems, natural and agricultural, have two major processes: a flow (of energy) and a cycle (of nutrients). Both processes allow the linking of ecosystem components and are driven by the Sun’s energy. Ecosystems in natural equilibrium have open energy flow, with energy being exchanged across the boundaries of most systems in the form of solar energy and heat. In natural ecosystems the only source of energy is the direct insolation, and the energy is fixed by photosynthesising producers in the form of plant glucose. Conversely, agro-ecosystems have numerous indirect inputs of energy, and farmers import energy in order to maximise the growth and yield from crops. Examples include the energy consumption of fuel for farm machinery, the use of energy in the industrial production of agro-chemicals and the electrical demand for greenhouse heaters and grow lamps. The great majority of imported energy is sourced from fossil fuels, and most agro-ecosystems are reliant on industrial processes to maintain outputs. In addition, many organic products of agriculture, for example manure or hay are exported away from the system for use elsewhere. This results in far less energy rich dead organic matter (detritus) in agro-ecosystems, unlike natural ecosystems where energy is contained within the system, decomposed by fungi and bacteria, and recycled through plant growth. Natural energy flows consist of complex food webs, in which feeding patterns of organisms are interconnected, and numerous organisms may feed at a certain trophic (feeding) level. For example in oak woodland many insect and caterpillar species feed on oak trees, and numerous bird species (i.e. robin, blue tit, woodpecker) feed on the same insects and are in turn consumed by predators such as the sparrow hawk. With so many different consumers on each level energy is dispersed across a low density of many different species, and the energy held by each species in the ecosystem is small. The inefficient transfers result in up to 90% of energy being lost at each trophic level interface. However, in agro-ecosystems human farmers rationalise food webs to chains with as few trophic levels and consumers as possible, in order to limit energy loss and obtain maximum energy yield. This is achieved through monocultural farming and pest control. Most efficient is arable farming, and potatoes can convert an average 0.25% of insolation received into crop yield, this compares to only 0.02% of energy converted to yield with beef herds feeding on grass.
Nutrient cycling in natural ecosystems is via three main pathways (see below) between the three stores of the litter, biomass and soil. There are some exchanges with external environment (i.e. leaching – see below), but in a system unaffected by man all dead organic matter is returned to the soil (fallout pathway), and so the system is at this stage effectively closed.
Phosphorus and potassium are both essential nutrients for plant growth, but nitrogen is required in the highest concentrations in the soil. The recycling of nitrogen through its complex cycle maintains the equilibrium in natural ecosystems between nitrate uptake and deposition. However, in agro-ecosystems over cropping and export of crops quickly exhausts accumulated nitrates, and so farmers must rapidly subsidise the soil with fertilisers, either inorganic (i.e. NPK), or organic (i.e. cattle manure or mulch) in order to maintain adequate nitrate concentrations. Another method of nitrating the soil is through the growth of leguminous, nitrogen fixing crops such as clover or beans, which fix nitrates from the nitrogen gas in air. This speeds up the rate of nutrient cycling in order to accommodate for the high rate of plant growth in commercial crops, and natural ecosystems have a far slower turnover of nutrients. The main disparity between the nutrient cycles of natural and agro-ecosystems is in the path taken by nutrients fixed in crops. Without intervention the majority is returned as dead organic matter, but in agro-ecosystems the great majority of the biomass is exported as crops, fodder crops or forage, and usually only a small fraction of the biomass on farms may be left to seed.
Not all farming methods require such drastic alterations to natural cycles, and shifting cultivators, nomadic pastoralists and hunter-gatherers use extensive agricultural techniques that mimic sustainable natural systems, and are successful in supporting small, low-density populations in places such as the Amazon rainforest and Papua New Guinea. Nonetheless, the majority of the human population relies upon intensive settled agriculture, and carefully controlled agro-ecosystems. The environmental impact of such reliance is significant, with even natural ecosystems experiencing feedback from the action of farmers.
Examine the environmental impact of intensive farming
Agriculture has had a widespread effect on every aspect of the environment, and since 1945 traditional agriculture in the U.K and in industrialised countries worldwide has become increasingly intensive. The means by which such reliable high yield crops, with no requirement for crop rotation are maintained is primarily through the application of a cocktail of agro-chemicals, and each chemical family has a range of impacts on the environment.
Fertiliser use is possibly most significant, as the relative fertiliser use is much greater than any other artificial supplement to crops, and application has increased five-fold since 1930 due to the trend towards intensive productivist agriculture. The countryside around Saint-Brieve and the Gouët basin in Brittany has been subject to severe negative effects of inorganic NPK overuse on vegetable cash crops since world war two. Runoff during rainfall is a major problem, with concentrations of nitrates and phosphates in the Gouët River reaching very high levels. The high nitrate concentration especially has resulted in eutrophication in tributaries and slower moving waterways, with abnormally rapid algal growth. The loss of light and oxygen from the water as the algae decomposes is detrimental to aquatic life, and populations of threatened species such as newts and some aquatic invertebrates have been adversely affected. In addition the seepage of nitrates into groundwater near drinking water extraction sites renders the water unfit for drinking, as excessive nitrate intake may damage the development of pre-natal infants and bring on stomach cancers. Inorganic fertilisers do not add organic matter to the humic layer, and so intensive arable land has a very low organic content that results in the loss of soil structure. The consequence of this is reduced field capacity, and so a worse response to shock rainfall events, often leading to rapid overland flow. Furthermore the soil is more prone to water and wind erosion, possibly leading to gullying, and the nutrient retention capacity of the soil can be significantly decreased. Albourne farm in the South Downs has experienced rift and gulley soil erosion that has been the most rapid in the U.K (in excess of 200m3 per year). This was the result of the lower organic content from the overuse of NPK fertilisers combined with the thin chalk soil, use of heavy machinery and ploughing of sheep pastures for arable crops. Chemical fertilisers can also encourage the thriving of weeds and grasses, which reduce biodiversity in native meadow wildflowers species (i.e. orchids). Overuse and excess nitrates can lead to acidification of the soil as the release of ammonia results in sulphate deposition. Furthermore, fertilisers and other agrochemicals contain impurities. As a result heavy metals (i.e. lead, zinc and arsenic) and trace elements may accumulate in the soil and reach harmful levels toxic to some plants and organisms. Even use of organic fertilisers, in the form of slurry, can have an impact when carried out to excess. In the Netherlands there is a surplus of manure from dairy farms and livestock rearing. 94 million tonnes are produced per year, but only 50 million tonnes can be safely absorbed by the soil. The adverse effects are similar to those caused by inorganic nitrates and phosphates when leached and washed into water supplies.
Pesticides are another widespread agro-chemical, and as environmental poisons they can pose a threat to local soils and wildlife. This has been a significant problem to the ecology of the British Isles, where 95% of cropped land is sprayed. Bioaccumulation of pesticides such as DDT can build up in the tissues of living organisms that feed on invertebrates until they become toxic. This has been held responsible for the massive reduction in native farmland bird species in the U.K such as the Fieldfare and Corn Crake over the past fifty years. Beneficial soil organisms involved in nutrient cycling and organic decay can also be killed by the toxic chemicals, and natural pest predators are reduced in numbers by the non-selective poison. In the Paris basin 800 species of fauna, especially bees and butterflies have been significantly reduced in numbers. As with fertilisers, pesticide runoff into waterways can impact negatively upon aquatic ecology, and aquatic ecosystems are often more fragile than those on land. In the U.K a rapid decline in frog populations is thought to be partly attributable to pesticide use. Moreover, intensive use of pesticides produces pesticide resistant pests through natural selection, and these pose an even greater threat to crop production. Herbicides used to clear weeds reduce the richness of floral species, and can remove the valuable meadow habitat for many invertebrate species when used extensively.
Soil erosion by water and wind is a problem in areas of intensive arable farming, and about 90% of farmland is losing soil above a sustainable rate. The average rate of soil erosion is eight times faster than the rate of soil formation. In England and Wales 37% of land is vulnerable to soil erosion, with areas of light soil (i.e. S. Downs, Yorkshire Wolds, Norfolk Fens and Breckland) most at risk. The Norfolk Fens have experienced very rapid erosion rates due to the large flat fields with few trees to act as windbreaks, and soil erosion can reach more than 17 tonnes per hectare on bare soil (compared to 0.68 tonnes on grass). Losses are greatest during prolonged dry spells and dust storms or ‘Fen blows’. Peat coverage was once up to 5 metres deep, but in places is now less than a metre and so thin that farmers are now ploughing the clay below. The South Downs has the problem of light chalk soils coupled with the cultivation of steeper upland slopes converted from grazing with thinner soil layers. Fen drainage in Norfolk and Netherlands and intensive arable cultivation by field expansion has also had a negative impact on biodiversity. The loss of dykes borders and willow trees has reduced the natural cover available for wildlife, and recent mechanical and herbicide use has kept vegetation away.
Changing management practises have sought to increase productivity, and amalgamated fields by the removal of hedgerows. The hedgerow is a valuable habitat for a rich variety of bird and mammal species (i.e. sparrow and hedgehog), and declines in populations over the past 50 years in the U.K correlate with intensifying agriculture.
Mechanised cultivation itself has an impact on the environment, and constant ploughing by heavy machinery may oxidise organic matter and break up soil crumbs. The result is a soil that may have a lower field capacity and be more susceptible to wind or water erosion. The rapid removal of nutrients, if exceeding the rate of nutrient replacement by fertilisers or farmyard manure can generally reduce soil fertility and hinder plant growth. The pressure of heavy tractors can compact and deform soil structures, and this is particularly pronounced with wet soils, when structural or particle bonding is weakest. Compaction in such a way reduces the size and amount of pore spaces in the soil, reducing aeration and drainage of the soil. This can cause high runoff and exasperate soil erosion in intensively farmed areas, and is a suggested cause for surface gulley erosion that is widespread in East Germany. Intensive industrial farming is also demanding of fuel, and farm processes contribute a significant amount to carbon dioxide emissions that may lead to global warming.
It could, however, be argued that with the exponential growth of the human population intensive agriculture in the long-term reduces the rate at which new areas of land have to be cultivated, effectively reducing the need for extensification and environmental damage. However, the opposing argument suggests that if farming was not only extensified, but also made sustainable, then the positive ecological gain would outweigh the loss of some natural habitats. Technology has provided few suggestions as to how intensive productivist agriculture could be replaced, and the protest against GM crops is narrowing down current alternatives. The impact of modern farming on the environment is certainly damaging, but it provides a tangible and potentially successful solution to the worlds hunger.